Leak rate testing is an engineering challenge where on the one hand, engineers must meet strict leak rate standards on a wide range of products and systems from semiconductor packages through medical product packaging to chemical storage vessels and liquid/gas handling systems. On the other hand, they have to make the leak testing process low cost and independent of operator whilst in many applications making the process automated and fast as this step may otherwise become a manufacturing bottleneck. To address these conflicting demands engineers must understand all aspects of the leak testing process. It is also important that it is understood what a product leak is. This common term is not always well defined but is basically a material flow from or into a product (a control volume) within a predetermined period of time which is in excess of an allowable limit. Product leaks are typically caused by open flow paths, such as pinholes, broken seals or material porosity and in most cases the product leak is a very small flow. It is this process of quantifying a product leak and eliminating products based upon the measured leak that is known as leak testing. In the pharmaceutical, medical, and food industries, it is called package or seal integrity testing.
Leak testing requires the accurate measurement of very small flow rates of a gas or liquid within what may in some instances be made for a large volume or in others rapidly. Typically measured as a flow rate, such as standard cubic centimeters per minute (sccm) or cubic centimeters of helium per second (cc/s He) and millibar liters per second (mbar.l/s) and may, according to application, range from 10−3 to 10−12 mbar.l/s. In some cases, the leak flow rate is correlated to a “virtual pinhole,” to quantify the size of potential defects. For example, to prevent contamination, a sterilized medical package must be sealed such that a “virtual pinhole” in the product is smaller than the size of the smallest microorganism (commonly 0.2 μm in diameter). This theoretical pinhole dimension and the leak flow rate are correlated to each other.
Irrespective of the actual component, device, product and/or system being leak tested the balance of speed, accuracy, and cost exists. Whilst increased speed (reduced time) of leak testing reduces cost per unit a corresponding reduction in accuracy from this may lead to increased costs from yield (as rejections may actually have passed with an increased accuracy test) and/or product failures and customer impact (as products failing at the customer which were incorrectly passed impact yield, customer satisfaction, and in critical cases may lead directly to damages payable by the manufacturer. Accordingly there is considerable benefit to manufacturers in increasing accuracy, increasing speed, and increasing defect detection in manufacturing leak testing.
Additionally some scenarios present further issues, such as for example large volume leak testing. Leak testing with large part volumes in the 30 L (˜7.9 gallons) and above range, creates additional challenges including temperature sensitivity and pressure sensitivity. Take the example of a 77 L part, approximately 19 gallons, then a conventional flow based leak test would uses a flow meter in series with a pressure regulator such as shown in FIG. 1. The Test Pressure Regulator R2 115 keeps the pneumatic pressure at the desired test pressure and the Flow Sensor F1 130 measures the flow to the Device under Test (DUT) 140. The test pressure regulator 115 is typically vented to atmospheric and this forms the basis for the test pressure. For example if the atmospheric pressure is 14.7 psi and the regulator is set to 7.5 psi the absolute test pressure is (14.7+7.5)=22.2 psi. Accordingly as the atmospheric pressure varies so does the absolute output pressure of the regulator.
Referring to Table 1 there is shown the resulting flow measurement if the part volume is 77 L and the atmospheric pressure varies by 0.002 psi. The flow meter will have to allow 6.9 cc to flow in order to allow the test chamber pressure to change. This is a large error if the goal is to read down to flow rates of 2 sccm reliably.
TABLE 1Calculation of Volume Change due to Atmospheric Pressure ChangeDescriptionValueUnitAtmospheric Pressure Change0.002psiVolume of Part77LTest Pressure7.5psiP absolute14.7psiVolume Change due to Atmospheric Pressure6.9CcChange
As depicted in FIG. 1 the Test Pressure Regulator R2 115 and Flow Sensor F1 130 form part of an overall pneumatic circuit with DUT 140. Air is coupled from a source, commonly referred to as the shop air, filtered with filter 105 and pressure regulated with Regulator R1 110 before being coupled to the Test Pressure Regulator R2 115. The output of the Test Pressure Regulator R2 115 is coupled to the Flow Sensor F1 130 via Supply Valve V3 120 and Flow Test Valve 125 and therein after the Flow Sensor 130 to DUT 140 wherein an Absolute Pressure Sensor P1 135 is also coupled together with a Calibration Orifice 145 allowing calibration to be performed via a non-return valve. The output of the Supply Valve V3 120 is also coupled to Fill Valve V1 150 and Exhaust Valve V2 155 which is also coupled to the output of the Flow Sensor 130 and Exhaust 160.
Accordingly through the appropriate sequencing of these valves the DUT 140 may be filled, pressurized and tested before being exhausted to air for de-coupling and another DUT 140 attached. Referring to FIG. 2 the resulting flow output (sccm) and ambient pressure (×2000 and DC removed) are depicted for such a DUT with volume 77 L over a period of 2000 seconds.
Such issues combined with the continued drive for more accurate leak test results, due to increased attention to quality, means that new control/measurement techniques are required. Accordingly embodiments of the invention address manufacturing requirements by providing for high accuracy flow based leak testing of large volumes, providing adaptive techniques for use during testing, providing equivalent circuit modeling techniques allowing optimization and parameter extraction to be simulated prior to manufacturing commitment, and providing for the automatic tuning of setup parameters.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.